Conversation list is blank after selecting a folder for a while
After selecting a folder, the conversation can remain blank when the IMAP connection used for it has gone bad due to a flaky network connection, etc, and Geary is waiting for the remote list/etc commands sent via it to time out (currently 30s).
We should be passing LOCAL_ONLY to the ConverationMonitor, but then that might break retrieving partial messages from the remote that don't meet the monitor's, ConversationList's or ConversationViewer's requirements. In these cases, either the conversation should not be shown when offline (or shown with a placeholder), or else the monitor should then explicitly go fetch the full messages needed from the remote.